The present invention has broad applications in surgery and other medical procedures for ablation, i.e. removal of obstructive or unwanted tissue, by tissue vaporization. One important application of the invention is for treatment of prostate enlargement or benign prostatic hyperplasia (BPH). BPH is a common condition in men over the age of 50 that occurs when nodular tissue from the prostate gland grows into and obstructs the urethra. BPH is characterized by difficulty urinating and a variety of other related symptoms.
Transurethral resection of the prostate (TURP) has been the most common surgical procedure for BPH. A resectoscope is inserted into the penis through the urethra and up to the prostate gland and an electrically heated wire loop is used to remove tissue from the interior of the prostate gland. TURP is considered by some to be the “gold standard” in treatment of BPH because it provides reliable symptomatic relief and can be used in large, as well as small prostate glands. However, there are significant drawbacks to the procedure. TURP is performed using spinal or general anesthesia and a 1-3 day hospital stay is generally required. A urinary catheter must be left in place for at least 1-3 days after surgery and the recovery time is typically four to six weeks. The known side effects of TURP include excessive bleeding, frequent urge to urinate, retrograde ejaculation, erection problems, painful urination (dysuria), recurring urinary tract infections, bladder neck narrowing (stricture), and blood in the urine (hematuria).
For these reasons, recent efforts have been focused on developing less invasive methods of treating BPH, including various methods of laser prostatectomy. The research goal has been to develop methods that are as effective as the “gold standard” of TURP in relieving symptoms, but are less traumatic to the patient and have fewer side effects.
One known method of performing laser prostatectomy involves using a laser for coagulation of the enlarged prostate tissue. Using a fiberoptic laser delivery device, the tissue to be removed is coagulated to kill the tissue. In one variation of this procedure, the laser energy is directed at four regions of the prostate tissue designated as the 2, 4, 8 and 10 o'clock positions. The tissue coagulation results in an immediate swelling of the surrounding tissue, therefore a catheter is allowed to remain in place for several days following the operation to allow for drainage of urine. Once the swelling subsides, the catheter is removed and over a period of several weeks the dead tissue sloughs off naturally, leaving an open passage through the urethra. Although this approach has been shown to be effective, it has the distinct disadvantage that the results are not immediate. The patient must endure the discomfort and inconvenience of having a catheter placed in the urethra for a number of days. In addition, some patients will experience continued dysuria or an inability to void after the catheter is removed.
Because of the shortcomings of the laser coagulation approach, recent efforts have been directed toward developing a method called photoselective vaporization of the prostate (PVP). Theoretically, if the enlarged prostate tissue can be completely removed at the time of treatment, then the patient should experience immediate relief from many of the symptoms. One laser that has been evaluated for this procedure is a frequency-doubled Nd:YAG laser. The 1064 nm beam of a Nd:YAG laser is directed through a nonlinear optical element, such as Potassium Titanyl Phosphate (KTiOPO4 or KTP) or Potassium Dihydrogen Phosphate (KDP), which absorbs the laser radiation and reemits it at twice the frequency (that is, half the wavelength) resulting in a 532 nm visible green light beam.
The 532 nm beam of the frequency-doubled Nd:YAG laser has a high absorption in the oxyhemoglobin component of blood. Since blood is the target chromophore of the 532 nm wavelength, the first pass of the laser results in ablation and carbonization of the surface tissue. However, the underlying tissue is devascularized, resulting in reduced ablation efficiency of the 532 nm wavelength on subsequent passes of the laser. From the procedural point of view, after the first pass using a 532 nm wavelength laser for BPH, the tissue blanches and it becomes increasingly difficult to vaporize additional tissue. Completion of the procedure will require an increase in the power setting of the laser, if more power is available, or will require more procedural time at the lower tissue ablation rate. Various scientific and clinical papers have reported that, as a result of the decreased ablation efficiency, 532 nm wavelength laser systems do not perform well with large prostate glands greater than 50 gm. For example, Tugcu et al. reported that in a series of 100 patients with prostate glands ranging from 74-170 ml, a procedure time of 100-240 minutes was required for ablation using an 80 watt “KTP laser” (Urologia Internationalis 2007; 79:316-320).
The efficiency of the laser system at vaporizing tissue is also adversely affected by fowling of the fiber tip with tissue, char or other material. Once the fiber tip has been contaminated, the temperature of the fiber will quickly rise with added laser energy and thermal runaway could result in damage or destruction of the fiber. For this reason, the 532 nm wavelength laser is recommended only for non-contact vaporization of the prostate. Yet, at the same time, for effective tissue vaporization, the fiber tip must be maintained a distance of approximately 1 mm or less from the tissue surface without contacting it. In practice, this is quite difficult and requires a great deal of training and practice on the part of the surgeon.
Others have reported using a 100 watt holmium laser to treat BPH in a procedure called Holmium Laser Assisted Prostatectomy, or HoLAP. The Holmium laser at 2100 nm is highly absorbed in water, and it will ablate any tissue with even a small amount of water contained in it. Water exists in all cells. Holmium laser treatment for BPH is conducted with water as an irrigant; therefore the laser energy has to pass through water to reach its intended target. Thus, a significant amount of laser energy is lost just getting the beam to the prostate tissue. On the plus side, the extremely high absorption of the 2100 nm holmium laser energy by water means that almost all of the laser energy that reaches the tissue is used in ablation or vaporization of the tissue. Very little energy is left over to cause thermal damage and coagulation in surrounding tissue. This leads to what holmium researchers refer to as the WYSIWYG (What you see is what you get.) effect, meaning that the result seen through the cystoscope at the end of the procedure is in effect the final result because there will not be a significant amount of tissue sloughing off later due to coagulation. However, the extremely high absorption of the 2100 nm holmium laser energy at high peak power combined with the pulsed delivery also results in what some doctors have referred to as the “clam chowder” effect. The tissue gets chewed up by a multitude of tiny explosions within the tissue. After the first pass with the laser delivery device the tissue surface is pocketed with ablation craters, therefore a higher and higher percentage of the laser pulses is directed into a crater and is absorbed by the irrigation fluid so that it never reaches the tissue, which reduces ablation efficiency. In addition, while these tiny explosions are ablating tissue they are violent enough that bleeding occurs and, since there is not much tissue heating, there is not enough coagulation to control bleeding well. Additionally, while the holmium laser ablates tissue very well regardless of the presence of blood in the gland, it does so at significantly lower tissue penetration depth and lower tissue vaporization rate than the 532 nm laser, requiring even longer procedure times.
U.S. Pat. No. 5,057,099 issued Oct. 15, 1991 to John L. Rink entitled “Method for Laser Surgery”, which describes a fiber tip protection system (FTPS) for use with pulsed lasers, is hereby incorporated herein by reference in its entirety. Additionally, U.S. Pat. No. 5,092,865 issued Mar. 3, 1992 to John L. Rink entitled “Optical fiber fault detector” and U.S. Pat. No. 5,269,778 issued Dec. 14, 1993 to Rink et al. entitled “Variable Pulse Width Laser and Method of Use” are hereby incorporated herein by reference in their entirety.